Print Head Longevity Advice

There are a lot of factors that determine what the final recommended voltage settings will be. There is no "Normal". The major factors that play a role in determining the jet voltage are; Piezo electrical characteristics, orifice size, material properties such as surface tension and viscosity, piezo poling efficiency, potting process stability, etc. Any change in any of the above will result in a different drive voltage setting. My jets are made with a different piezo than Solidscape's (SS). My potting process is also different. My orifice size is better regulated than SS. I know all of this because they are using a process that I developed 25 years ago. They have made some changes, but, fundamentally, it is the same. The bottom line is my considered opinion that Solidscape jets that have an initial voltage setting of 65/32, is only driving the piezo with 33 volts. On the surface this looks like you have lots of voltage to adjust the print head with. Not true. What it tells me is that the orifice is too large and it is creating droplets that are also too large if the jet was run at 65/20. In order to reduce the mass output of the jet, they have to reduce the actual jet voltage (6530) in order to reduce the drop size to meet the config file requirements. If you were to reduce the jet voltage on a Solidscape print head just a couple of volts lower than their recommendation, such as 65/36, the jet would stop working. Also, because the jet is running at such a low voltage, the velocity of the drop is low and unstable. With my jets, the orifice is intentionally a little smaller. This by itself requires a little more voltage to make a drop the same size as a Solidscape print head and therefore requires more difference in the two numbers, IE 65/21, instead of 65/28. More voltage from a smaller orifice means that the droplet will be traveling at a higher velocity at the same mass. This creates a much more stable droplet. When comparing a new Solidscape jet with their recommended setup, to my jets with my recommended setup, Solidscape's typical setup will be somewhere around 65/30, whereas my setup will be more like 65/20. This difference has absolutely NO reference to the longevity of the print head or to its efficiency. This is simply a starting point and for a new jet it tells me that the operator will be able to throttle the output more effectively with my jet than with a Solidscape jet.

When the jet settings begin to falter, or fall off from their original settings not due to thermal damage, ONLY indicates that the piezo is losing some poling efficiency. This is 100% normal and has no bearing on the quality of the print head. Simple recalibration will re-establish the output to the proper level. As time goes on the jet will continue to depole and need to be re-calibrated periodically. It is an unavoidable time/temperature phenomenon. Eventually, it will get to the point where the voltage will be something like 65/5, at which point the 65 will have to be changed to 85, and the 5 to 25. If the previous voltage was 65/5, then the new voltage will start out at 85/25. This will give the jet 20 more volts of life and delay the need for repoling. Ultimately, the jet will need to be repoled. Depending on the heat history of the print head and the number of "Hot Purges" that it has endured, simple repoling will be sufficient to restore the jet to near new condition. Recalibration will then set the voltage back to 85/45, or 65/25, which is the same thing.

It is extremely important to NOT damage the potting compound. Such damage is permanent and irreversible. This will only happen if the jet temperature is raised above 125 degrees C. Each and every time that the jet is raised above 125 C, there is a little damage that occurs for each minute that it stays at this higher temperature. The higher the temperature, or the longer the exposure time, the more damage there is that occurs. Eventually there will be enough accumulated thermal damage so that the print head will no longer function and will need to be totally rebuilt. Simply limiting the exposure to high temperatures will prolong your print head life significantly. I strongly recommend NEVER going above 123 C because you can't depend on the precision of the temperature board calibration. Keeping your print head clean limits the chances that particles will get inside and cause a clog. Keeping the temperatures under strict control will eliminate thermal damage but, there is nothing you can do to keep the piezo from depoling. However, simple re-poling will restore the print head if it is not otherwise damaged. So, if the print head stops printing because of a clog or other reason, under no circumstances do you want to initiate a "Hot Purge". Resorting to desperate measures will only cost more money and will accomplish very little in the long run. It is better to return the print head for relatively inexpensive servicing rather than resort to these radical measures. However, those who do resort to these desperate and radical measures, I say "Thank You". I would rather sell a newly refurbished jet than to repair a damaged one. The above advice is for your benefit, not mine.


Operating Instructions for the BFT Piezo Repoler


Thank you for purchasing our Piezo Repoler. You have made a decision that will add life to your Solidscape print heads and time to your work schedule. It is recommended that you repole the print head piezo at least twice each year. But remember, if it ain't broke, don't fix it. Before resorting to repoling your print head's piezo, first try to remedy the problem by recalibrating the print head.


The repoler is simple to use. It is automatically in the off mode until you press the “MEAS” button. After releasing the button, the display will remain on for approximately 15 seconds. While the button is pressed, it is very important not to touch any portion of the Adaptor card because  there is 500 to 1000 volts DC available at the Adaptor card circuit. While the DC current is very low, it is still possible to receive a significant electrical shock by touching it.


There are 6 pictures labeled Fig. 1 through Fig. 6 that depict the proper installation of the adaptor card and the correct manner to mount the print head.

Step 1
Locate the BFT Piezo Repoler Adaptor as shown in Fig. 2. Observe the + and the – signs that are etched in the terminal board near the banana plugs. Make sure that the plug labeled with the + sign is inserted in the hole at the top end of the device with the RED ring around it. Insert firmly as shown in Fig. 3.

Step 2
On the back of the unit is the battery cover held in place with a small screw as shown in Fig. 4. Remove the screw being careful not to lose it. It is not a captured screw and can get lost very easily. Carefully slide the cover back towards the bottom end of the device and remove it. Insert the batteries as indicated and replace the cover.

Step 3
Proper setup of the device is important for correct readings in the display. Test the device before attaching a print head by pressing the “MEAS” button and observe the display. If the reading is 0.L, it is in the correct resistance mode, see Fig. 6. If it displays 0L, press the small black button under the 2000 M ohm label. Press it only once. Now press the “MEAS” button again a see if it reads 0.L correctly.

Step 4
Before repoling your print head, place it on your Model Maker until it has reached a proper operating temperature. Have the repoling device ready. Remove the print head from the machine and place on the Piezo Repoling Adaptor connector located near the rear edge of the board as shown in Fig. 5. It is important to repole the piezo while the print head is at or near its operating temperature.

Step 5
Press and hold the “MEAS” button for 15 to 20 seconds. Make note that the small indicator light is on and bright during this portion of the procedure as shown in Fig. 6.

Note:
While the button is pressed, observe the display screen. When the button is first pressed it is common to see some numbers appear in the display for 1 or 2 seconds. However, if numbers appear in the display and stay there for more than 5 seconds it indicates that your piezo has an electrical short. The display will read either 0.L, or 0L. The difference between the two indications depends on whether the unit is in 20 M ohm (0.L), or 2000 M ohm(0L) mode. Neither of these modes will have an affect on the poling function, however, to properly read the display and to determine if your piezo is shorted, the device should be in the 20 M ohm mode which is the 0.L indication. To change from one mode to the other, press the button under the 2000 M ohm label until the 0.L is displayed. This will only change during the 15 seconds that the device is turned on. To check if it is then in the proper mode, press the button again.  A print head with a shorted piezo must be sent back to BFT for repair.











Fig. 1, Repoler Device                                              Fig. 2, Adaptor Card                      Fig. 3, Proper Adaptor Card Insertion












Fig. 4, Battery Cover Removed                               Fig. 5, Print Head Mounting                                  Fig. 6, Correct Display